1. Field of the Invention
This invention relates to a spectral characteristic measuring apparatus such as spectral calorimeters, and more particularly to technology of correcting a shift of spectral sensitivity of a light sensor in a wavelength direction (hereinafter, referred to as “wavelength shift”) resulting from wearing of the apparatus with time or a change in ambient temperature around the apparatus.
2. Description of the Related Art
Generally, spectral characteristic measuring apparatus such as spectral colorimeters have amplitudes, central wavelengths, and half bandwidths of spectral sensitivities of a light sensor thereof calibrated, at the time of production thereof, with use of bright line spectra emitted from a laser or a reference color plate under a temperature control prior to shipment. However, measurement precision of the apparatus is likely to deteriorate if the spectral sensitivity of the light sensor is varied in the amplitude direction or wavelength diffusing direction after the shipment due to wearing of the apparatus with time or an ambient temperature change around the apparatus.
Generally, a light sensor has an arrangement such that a plurality of photoelectric conversion elements are arrayed at a certain interval in the wavelength diffusing direction of a spectral device so that each photoelectric conversion element receives light in a different wavelength band to output an electric signal corresponding to the received light intensity. If the light sensor is displaced relative to the spectral device in the wavelength diffusing direction due to wearing of the apparatus with time or the like, the spectral sensitivity of the light sensor may be shifted in the wavelength diffusing direction.
A shift of the spectral sensitivity of the light sensor in the amplitude direction can be corrected easily and a sufficient number of times by customarily performing white calibration with use of a white color plate for calibration each time before actual measurement, thus enabling to perform correction under environmental conditions for actual measurement.
Regarding a wavelength shift correction, according to the current state of art, it is a general practice to correct a wavelength shift of the spectral sensitivity of the light sensor by measuring spectral characteristics of a reference color sample whose spectral reflectance is known and by presumably calculating a shift amount based on a difference between the measurement result and the known data.
Many of the reference color samples, however, have large temperature dependency characteristics. Particularly, high chromatic reference color samples of colors such as red, orange and yellow which have steep spectral characteristics and thus are effective in correcting a wavelength shift have a temperature dependency characteristic such that a wavelength shift of 0.1 nm or more is generated with respect to a temperature change of 1° C.
In view of the above, in order to accurately perform correction with use of such a reference color sample, a) correction is performed at the same ambient temperature as the temperature at which the known spectral reflectance has been obtained, or b) the temperature of the reference color sample at the time of correction is measured, and a temperature correction is performed with respect to the known spectral reflectance.
In order to implement the former technique a), it is required to perform correction in a room under a strict temperature control, which raises the costs for correction. In case where the measuring apparatus is used in a site whose ambient temperature is different from the temperature at which the correction has actually been performed, there is no sense of performing such a correction. According to the latter technique b), it is required to measure a temperature dependency characteristic of a reference color sample having a certain spectral reflectance in advance and also to measure the temperature of the reference color sample at the time of correction for correction of the known data, which requires time and labor. In particular, since a portable spectral colorimeter is generally not used in a testing room but is used in a variety of environments whose ambient temperature differs, none of the above correcting methods is practical.
In addition to the above drawbacks, the conventional art requires measuring spectral characteristics of the reference color samples themselves and providing a space for storing these reference color samples. In view of the above, it is a general practice to return the apparatus to the manufacturing factory for correcting wavelength shifts of the spectral sensitivities of the apparatus, and such correction is seldom performed on the user side.
As mentioned above, wavelength shift correction of the spectral sensitivity of the spectral device is not performed on daily-basis in view of the cumbersome operations mentioned above, thus becoming one of the primary factors which hinder acquiring consistent measurement data.